The present invention relates to thin film heterojunctions photovoltaic cells formed from compound semiconductors, especially cells having copper indium diselenide (CIS) as a first semiconductor layer, and more particularly to such cells in which the semiconductor forming the light receiving face comprises primarily zinc oxide (ZnO).
A known photovoltaic cell structure which is believed relevant to the present invention comprises a first layer of copper indium diselenide forming a junction with one or more layers of cadmium sulfide. The background of such cells is discussed in considerable detail in U.S. Pat. No. 4,335,266 issued to Mickelson et al on June 15, 1982, which patent is hereby incorporated by reference for all purposes. As generally discussed in that patent, this type of solar cell is believed by many to be a viable alternative solar cell. The efficiency of such compound semiconductor cells have been gradually increased as various improvements have been made. In addition, they offer the opportunity of reduced manufacturing cost.
The basic improvement taught by Mickelsen involves a new method for forming the copper indium diselenide layer so as to not have an adverse affect upon the cadmium sulfide layer when deposited. Mickelsen deposits this CIS layer in two distinct regions with the first having an excess of copper and the second having a deficiency thereof. Diffusion between the two layers is said to achieve a uniform CIS structure while avoiding defects such as pure copper nodules at or near the surface upon which the cadmium sulfide is to be deposited. Such copper nodules, and possibly other defects, can cause short circuiting of the later deposited cadmium sulfide layer to the back face electrode of the finished device.
Despite the improvements in the CIS layer claimed by Mickelsen, it has still been found necessary to deposit the cadmium sulfide portion of the cell in two distinct regions. The first is a 0.5 to 1.5 micron layer of essentially pure cadmium sulfide which is of n-type conductivity and high resistivity. The high resistivity character of this region is believed necessary to limit the effect of defects such as pure copper or metallic copper selenide nodules in the CIS layer. The cadmium sulfide layer is then completed by deposition of a 2 to 4 micron thick layer of cadmium sulfide which has been doped appropriately to provide low resistivity.
While various improvements have been made in such CIS-CdS cells, several basic problems remain. One problem is that the relatively thick cadmium sulfide layer absorbs a considerable amount of incoming light before it can reach the active junction area. This necessarily reduces device efficiency. In addition, it is well-known that cadmium is a highly toxic heavy metal. While there is no evidence that cadmium in the completed devices presents any danger to the environment or to the user, the material does cause certain hazards in the manufacturing process. Thus, manufacturing costs are increased by the extra time and equipment which must be employed to avoid any danger to employees manufacturing devices incorporating cadmium and to avoid any environmental damage.
Some of the problems inherent in use of CdS have been avoided in experimental devices in which a heterojunction has been formed between CIS and ZnO. See for example: "A ZnO/p-CuInSe.sub.2 Thin Film Solar Cell Prepared Entirely by Spray Pyrolysis", M. S. Tomar and F. J. Garcia, Thin Solid Films, 90 (1982), pp 419-423; and "Chemical Vapor Deposited Copper Indium Diselenide Thin Film Materials Research" Final Report, March 1984, SERI/STR-211-2247. In the Tomar publication, ZnO was deposited on a SnO.sub.2 coated glass substrate by spray pyrolysis at a temperature of from 350.degree. C. to 550.degree. C. After similar deposition of a CIS layer, photovoltaic response with about 2% efficiency was measured. In the SERI report, efficiency in the range of 2% to 3% was achieved by ion-beam sputtering of ZnO films onto CIS films deposited by a close space chemical vapor transport technique. Thus neither of these references indicates that commercially practical efficiency can be achieved when ZnO is used in place of CdS in a thin film CIS heterojunction cell.